Asymmetrical Interactions betweenWolbachiaandSpiroplasmaEndosymbionts Coexisting in the Same Insect Host

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Abstract
We investigated the interactions between the endosymbiontsWolbachia pipientisstrainwMel andSpiroplasmasp. strain NSRO coinfecting the host insectDrosophila melanogaster. By making use of antibiotic therapy, temperature stress, and hemolymph microinjection, we established the following strains in the same host genetic background: the SW strain, infected with bothSpiroplasmaandWolbachia; the S strain, infected withSpiroplasmaonly; and the W strain, infected withWolbachiaonly. The infection dynamics of the symbionts in these strains were monitored by quantitative PCR during host development. The infection densities ofSpiroplasmaexhibited no significant differences between the SW and S strains throughout the developmental course. In contrast, the infection densities ofWolbachiawere significantly lower in the SW strain than in the W strain at the pupal and young adult stages. These results indicated that the interactions between the coinfecting symbionts were asymmetrical, i.e.,Spiroplasmaorganisms negatively affected the population ofWolbachiaorganisms, whileWolbachiaorganisms did not influence the population ofSpiroplasmaorganisms. In the host body, the symbionts exhibited their own tissue tropisms: among the tissues examined,Spiroplasmawas the most abundant in the ovaries, whileWolbachiashowed the highest density in Malpighian tubules. Strikingly, basically noWolbachiaorganisms were detected in hemolymph, the principal location ofSpiroplasma. These results suggest that different host tissues act as distinct microhabitats for the symbionts and that the lytic process in host metamorphosis might be involved in the asymmetrical interactions between the coinfecting symbionts.